test(webmcp-polyfill): add browser-mode Vitest config & declarative DOM tool support

[MiguelsPizza]

Motivation

• The polyfill tests exercise browser globals (navigator, document) and need a browser-mode Vitest path to run conformance-style tests reliably.
• Support declarative tool registration in the DOM so the polyfill can be validated against script/data-attribute based tool registrations used by consumers.

Description

• Added a browser-specific Vitest config packages/webmcp-polyfill/vitest.browser.config.ts wired to Playwright/Chromium and added test:browser / test:browser:ui scripts in packages/webmcp-polyfill/package.json so browser-mode runs are explicit and opt-in.
• Added browser testing dependencies to packages/webmcp-polyfill/package.json and updated pnpm-lock.yaml to include @vitest/browser, @vitest/browser-playwright, and playwright entries.
• Implemented declarative tool parsing and synchronization in packages/webmcp-polyfill/src/index.ts (parse scripts of type application/webmcp+json, [data-webmcp-tool] elements, startDeclarativeToolSync with MutationObserver, integration into initializeWebMCPPolyfill and cleanup logic, and exposing serialized cross-document results to the testing shim).
• Extended tests in packages/webmcp-polyfill/src/index.test.ts to assert parsing of declarative <script> entries and data-webmcp-tool elements and that declarative registrations are synchronized when DOM nodes are removed.

Testing

• Ran pnpm --filter @mcp-b/webmcp-polyfill build and the build completed successfully (tsdown/tsc step passed).
• Ran pnpm --filter @mcp-b/webmcp-polyfill test (Node-mode Vitest) and observed failures because the suite expects browser globals (navigator, document) in this environment; the Node-only run reported many failing tests (navigator is not defined).
• Ran pnpm --filter @mcp-b/webmcp-polyfill test:browser which starts Vitest in browser mode, but the run could not complete because Playwright Chromium is not available in this environment and playwright install chromium failed (browser download blocked by network: 403 Domain forbidden).
• Attempted pnpm --filter @mcp-b/webmcp-polyfill exec playwright install chromium and received a network/download failure preventing browser installation; this prevents executing the browser-mode tests here but the project is now wired so CI with an available Chromium (or allowed Playwright downloads) can run the conformance suite.

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Codex Task

[sentry]

Codecov Report

❌ Patch coverage is 87.09677% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
packages/webmcp-polyfill/src/index.ts	87.09%	11 Missing and 1 partial ⚠️

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